The Ashes Issue
Chemical-looping combustion is performed using oxygen-carrying materials such as metallic oxides that yield their oxygen in a reduction zone (referred to as “fuel reactor”) under suitable operating conditions. Once reduced, the material is carried to an oxidation zone (referred to as “air reactor”) where it is reoxidized on contact with an oxidizing gas (such as air or water vapour for example).
More generally, a chemical-looping combustion process comprises one or more reaction zones making up a reduction zone, wherein combustion of a fuel (a hydrocarbon-containing feedstock for example) is carried out by contact with an oxygen-carrying solid that is reoxidized afterwards in at least one oxidation zone by contact with air or water vapour prior to being sent back to the combustion (or reduction) zone(s). The reaction zones allowing chemical-looping combustion reactions to be conducted generally consist of fluidized beds or transported beds.
Chemical-looping combustion (CLC) of solid hydrocarbon feedstocks is a method allowing notably energy (vapour, electricity, etc.) to be produced by recovery of the heat released by the combustion reactions, while producing CO2-rich fumes. It is therefore possible to consider CO2 capture after condensation and compression of the fumes. It is also possible to consider the production of syngas, or even hydrogen, by controlling the combustion and by implementing the required purifications downstream from the combustion process.
In the reaction mechanisms associated with chemical-looping combustion in the reduction zone, it is established that the solid fuel goes through a gasification stage, promoted by the presence of water vapour or of carbon dioxide and by the temperature, then that the gas produced by the gasification stage is oxidized on contact with the oxygen-carrying material. If the solid fuel contains volatiles, the latter devolatilize at least partly on contact with the hot oxygen-carrying material and they are then oxidized thereby. It is also possible, in cases where the oxygen-carrying material naturally releases the oxygen according to the operating conditions, to have direct oxidation of the solid fuel by the gaseous oxygen released by the material in the fuel reactor.
Chemical-looping combustion of solid feedstocks requires severe and compelling operating conditions to be able to conduct the combustion reactions. In order to favour gasification of the fuel, high temperatures generally ranging between 800° C. and 1100° C., preferably between 850° C. and 1000° C., are necessary. The time required for gasification decreases as a function of the temperature and it generally ranges between 30 seconds and 30 minutes. It can therefore be advantageous to perform partial gasification, to separate the non-gasified fuel residue from the effluents and to recycle it. It is thus possible to reach rates of conversion (through gasification) per pass ranging between 50% and 80% in a temperature range between 850° C. and 1000° C., with reaction times ranging between 1 minute and 10 minutes, typically between 3 minutes and 5 minutes. The gasification times can be reduced by increasing the partial oxidizing gas (H2O, CO2) pressure.
Another problem linked with chemical-looping combustion of solid feedstocks relates to the formation of ashes. Indeed, solid fuels have not insignificant mineral material contents and, once combustion of the carbon and of the hydrogen is completed, solid residues called ashes form. Table 1 groups the analyses of two coals A and B by way of example. It can be observed that the ash content of the coals varies according to the origin of the solid feedstock, but this content is not insignificant. It typically represents 5 to 20% of the mass of dry coal. Some solid fuels such as pet coke have much lower ash contents. There are also solid fuels with higher ash contents.
These ashes essentially consist of silicon and aluminium oxide, but they also contain other components, as illustrated by way of example in Table 1.
TABLE 1Analysis of the various coals—Coal ACoal BDry coalAsheswt. %10.314.8analysisVolatileswt. %37.624Sulfurwt. %0.50.57Specific heatKcal/kg67106630UltimateCwt. %71.173.46analysisHwt. %4.773.87Nwt. %1.411.65Swt. %0.50.57Asheswt. %10.314.76O (by difference)wt. %11.925.69AshesSiO2wt. %6749.84compositionAl2O3wt. %19.240.78Fe2O3wt. %5.22.9CaOwt. %21.08MgOwt. %1.20.26TiO2wt. %0.91.96K2Owt. %1.70.64Na2Owt. %1.70.06SO3wt. %0.90.52P2O5wt. %0.21.05
The ashes resulting from the combustion of the coal are made up of residual fine particles. Their melting point varies according to their composition and it generally ranges between 1000° C. and 1500° C. However, at lower temperatures, for example between 800° C. and 1000° C., it is possible to observe a phenomenon of agglomeration of the ash particles that become sticky. They can therefore either agglomerate with one another, or they agglomerate with the particles of oxygen-carrying material. Considering the operating conditions in the chemical-looping combustion process, two types of ashes can be distinguished:                fly ashes: they correspond to the ashes that are carried into the fuel reactor by the combustion gases,        agglomerated ashes: they correspond to the ashes that agglomerate with one another or with the oxygen-carrying material and that are too heavy to be carried into the fuel reactor by the combustion gases.        
Fly ashes generally represent 50% to 99% of the ashes formed, typically 70% to 90%. Their grain size is relatively fine with generally at least 25% fines with sizes below 10 microns and 90% fines with sizes below 100 microns. The Sauter mean diameter representative of the fly ash grain size generally ranges between 5 and 30 microns, and it is typically close to 10 microns. The grain density of these ashes generally ranges between 2000 and 3000 kg/m3, and it is generally close to 2500 kg/m3.
The grain size of the agglomerated ashes is more delicate to estimate and depends on the conditions of implementation of the method. In general terms, the grain size of these ashes is estimated to be above 100 microns and their size can reach several millimeters.
Patent application FR-2,850,156 describes a chemical-looping combustion method wherein the solid fuel is crushed prior to being fed to the circulating fluidized-bed reduction reactor so as to allow more complete and faster combustion. The method produces nearly 100% fly ashes that are separated from the circulating oxides. Separation downstream from the circulating bed is first provided by a cyclone, then by a device comprising a bed fluidized by water vapour allowing separation of the unburnt particles from the metallic oxide particles. Entrainment of unburnt particles in the oxidation zone and therefore CO2 emissions in the oxidation reactor effluents is thus avoided. The fly ashes are separated from the oxide particles in a second circuit comprising a fluidized bed separator.
N. Berguerand's thesis “Design and Operation of a 10 kWth Chemical-Looping Combustor for Solid Fuels”, ISBN 978-91-7385-329-3, describes a device allowing coal combustion to be conducted using a chemical loop.
This device consists of an oxidation reactor using metallic particles, a cyclone allowing separation of the particles and of the depleted air after oxidation, a fluidized bed supplied with oxidized metallic oxides through the return leg arranged below the cyclone, wherein reduction of the metallic oxide is carried out by combustion of the coal. The coal is fed into the upper part of the dilute phase fluidized bed. In the reduction reactor, combustion of the coal takes place progressively: the coal particles first descend and devolatilize in the dilute phase, countercurrent to the fluidization gases, wherein the metallic oxides are present in small amounts only; then they come into contact with the fluidized metallic oxides in dense phase. The long residence time allows to gasify the coal and to produce combustion gases containing large amounts of carbon monoxide and of hydrogen that pass into the dilute phase.
According to this document, the reduction reactor is equipped with a particle separator integrated in the dense phase, which requires additional gas for the separation. In this system, no specific device allowing separation and discharge of the ashes formed during combustion of the solid feedstocks is provided.
In order to overcome the drawbacks of the two systems described above, the applicants have developed a chemical-looping combustion method allowing, even from coarse fuel particles, to obtain total combustion of the solid feedstock while minimizing the amount of solid feedstock to be recycled, which allows to maximize the energy efficiency of the method. The combustion method according to the invention allows to capture at least 90% of the CO2 emitted by the combustion in the fumes directly at the combustion reactor outlet, the capture rate being defined by the ratio of the amount of CO2 emitted in the fumes coming from the combustion reactor to the amount of CO2 emitted in the chemical-looping combustion process.
At the combustion process outlet, the CO/CO2 molar ratio of the fumes downstream from the cyclones is below 0.05 and the H2/H2O ratio is below 0.05. This is achieved, on the one hand, through optimization of the initial contact between the oxygen-carrying particles and the solid fuel so as to promote the coal gasification reactions and, on the other hand, through optimization of the contact between the gasification products and the metallic oxides so as to produce effluents that have undergone total combustion (H2, CO and HC<1 vol. % in the fumes).
Besides, separation of the unburnt fuel particles from the metallic oxide particles is carried out upstream from the reduction reactor fumes dedusting stage so as to best use the maximum kinetic energy of the fumes for separation of the two types of particles.
The chemical-looping method comprises at least:                contacting of the solid feedstock particles in the presence of metallic oxide particles in a first reaction zone operating under dense fluidized bed conditions,        combustion of the gaseous effluents from the first reaction zone in the presence of metallic oxide particles in a second reaction zone,        separation, within a mixture from the second reaction zone, of the gas, the unburnt particles and the metallic oxide particles in a separation zone,        reoxidation of the metallic oxide particles in an oxidation zone prior to sending them back to the first reaction zone.        
Downstream from the unburnt and metallic oxide particles separation zone, a dedusting system comprising for example one or more cyclone stages can be provided for separation of the particles carried along in the fumes of the combustion zone of the fuel reactor. The fly ashes are carried along in the fumes to this dedusting system with the unburnt solid fuel particles. In order to maximize the energy efficiency of the plant, it is necessary to recover the main part of the unburnt fuel particles and thus to carry out deep dedusting. This dedusting will then allow to recover the unburnt particles, as well as a large part of the fly ashes that are then recycled to the fuel reactor.
In order to remove the ashes, it is possible to position an enclosure comprising a fluidized bed on the line channeling the particles separated during the dedusting stage so as to eliminate the fly ashes through elutriation. However, this means does not allow to control elutriation of the ashes and elutriation of the unburnt particles separately. In fact, in this case, good elimination of the ashes produced can consecutively be translated into a significant elimination of the unburnt particles and therefore a decrease in the energy efficiency or a decrease in the CO2 capture rate.
Furthermore, in cases where particles of the oxygen-carrying material are carried to the dedusting zone, it is necessary to dimension a large-size fluidized bed so as to allow sufficient time for elutriation separation in the fluidized bed.
In order to remove the ashes produced in the reduction zone without eliminating too many unburnt feedstock particles, the applicant has developed an improvement in this method using a particular reduction zone configuration, with: a first reaction zone operating under dense fluidized bed conditions; a second reaction zone; a fast separation zone for separation of the unburnt solid feedstock particles, the fly ashes and the oxygen-carrying material particles within a mixture coming from the second reaction zone; dedusting of the fumes; a particle stream division zone, part of the particles being directly recycled to the first reaction zone, the other part being sent to an elutriation separation zone so as to recover the ashes and recycle the dense particles to the first reaction zone.
However, ashes can also be carried to the oxidation zone. Indeed, during combustion in the reduction zone, the feedstock degradation produces two types of ashes: either agglomerated bottom ashes that are not transported pneumatically with the entrained metallic oxides bed, or fly ashes that are carried along with the solid bed. Whereas the former ones are readily removed at the fluidized-bed reactor bottom by simple gravity flow, the latter flow together with the oxides and, in the absence of a device controlling their population in the bed, they accumulate therein. The ashes are thus carried from the reduction zone to the oxidation zone, all the more so since no ash population control device is arranged in the reduction zone.
The accumulation of ashes has many consequences and it generates three major drawbacks:                solid flow disturbance through modification of the average grain size of the bed likely to cause control problems as regards the circulation of this solid bed,        dilution of the oxygen-carrying function of the metallic oxides, leading to a decrease in performance of the method,        the risks of agglomeration between ashes or between ashes and metallic oxide particles are increased.        
Alternatively or complementarily to the removal of ashes and fines in the reduction zone, a new configuration is provided for the oxidation zone (or air reactor), which allows to use the kinetic energy of the air (oxidizing gas) available during reoxidation of the solid to achieve separation between the oxygen-carrying particles (metallic oxide) and the particles to be removed: ashes and metallic oxide fines, then to achieve separation of the ashes in an enclosure arranged on a solid circulation line, but in a zone where the solid unburnt particles are absent, i.e. downstream from R1 and upstream from R0.